With the increasing growth of semiconductor and microelectronic technology, optical materials have attracted considerable attention since they can meet the demanding performance requirements for telecommunication, data communication and information storage systems.[1,2] One of the most interesting and useful classes of optical materials is photo-reactive polymers, which undergo photo-transformation under direct ultraviolet (UV) or visible (Vis) light irradiation. Recently, the synthesis of photo-reactive polymers has developed to a widespread research area, due to their applications in photoresists,[3] microlithography,[4] holographic data storage,[5] and as photocurable adhesives in optical interconnections.[6]
There are several curable molecular and polymeric systems currently available, these systems usually contain reactive acetylene, epoxide/amine or anhydride groups, as curing agents, and benzocyclobutene (BCB). The problems or weaknesses associated with these curing systems are: high curing temperature is required; environmentally harmful gases are released during the curing procedure; large amounts thereof are required, thereby increasing the production cost and causing voids in molded articles; the area of application is confined due to the lack of interpolymer coupling reactions and only based on dimerization and Diels-Alder cycloaddition reactions with respect to the BCB chemistry. Above all, these curing systems are not best suited to the UV curing process, as they require the use of photoinitiators and photochemically generated acid or base catalysts.
The chemistry of most photo-reactive polymers used in photoresists is based on the incorporation of a functionally photosensitive moiety, for instance, carbon-carbon double bond,[6] epoxide,[6] Novolak/diazoquinone[4b] or photogenerated acid group[4b]. These systems need a photoinitiator, photosensitizer, or a photoacid generator to carry out the photoreactions. If a direct photocurable system consisting of a photo-reactive latent group were available, the cure formulation would be simpler and stable without at least a photoacid/base generator and the cost of fabrication process would be reduced. The incorporation of a latent photo-reactive group into a polymer allows the structural control and tailoring of inherent properties of the polymer.
Benzocyclobutenone (BCBO), that can be easily prepared in large quantities from anthranilic acid, homophthalic anhydride, or o-toluoyl chloride according to known methods[7], has been reported to undergo electrocyclic ring-opening upon thermolysis[8,9] or photolysis[8,10] to give a reactive ketene, xcex1-oxo-o-quinodimethane. Besides the facile thermal[4+2] cycloaddition reaction with dienophiles such as carbonyls,[11] and [60]fullerene[12], it has been demonstrated that the thermally-generated ketene (at 150xc2x0 C.) also undergoes a coupling reaction with a variety of alcohols to form esters in high yields[13]. An earlier patent (Z. Y. Wang, U.S. Pat. No. 5,869,693) teaches the preparation of a variety of functionalized benzocyclobutenones, particularly 5-aminobenzocyclobutenone, and the polymers derived therefrom. Further, it teaches thermally induced crosslinking and curing with the functionalized benzocyclobutenones.
If the reaction of BCBO with itself or alcohol could also be done under UV irradiation at ambient temperature, BCBO could thus be considered as a latent photo-reactive group, and used as a curing agent for UV-Vis curing of polymers.
There is a need for a UV curing agent that is latent reactive or thermally stable at ambient temperatures and up to 250xc2x0 C.
There is a need for having such a UV-Vis curing agent that can be easily functionalized and incorporated into a wide spectrum of polymers that are either known and in commercial production or disclosed in the literature.
There is a need for having such a UV-Vis curing agent that survives the polymerization conditions and remains inactive until being triggered by UV-Vis irradiation at ambient temperatures. There is a need for a UV-Vis curing process during which no volatile small molecules are generated.
There is a need for a new type of photo-reactive polymer capable of undergoing grafting and crosslinking by simple UV-Vis irradiation, without using any catalysts or photosensitizers. There is a need for a new type of photo-reactive polymer capable of providing a fine pattern in a submicron to centimeter resolution on different substrates, by simple UV-Vis curing processes without using any catalysts raid or photosensitizers.
The present invention relates broadly to a series of functionalized photo-reactive benzocyclobutenones that can be used for photochemically crosslinking a variety of hydroxy- or amino-containing polymers.
This invention also relates to a series of polymer compositions that undergo grafting or crosslinking by UV-Vis irradiation to produce polymeric products having an improved balance of properties, and which can be used for surface modification and photochemically forming patterned coatings on substrates.
In particular, the present invention discloses the functionalization of BCBO, the incorporation of BCBO as a latent UV curing agent in a wide spectrum of addition and condensation polymers, and the UV curing procedures and applications thereof of said polymers in surface modification and photopatterning according to their the photo grafting and photo crosslinking reactions.
In accordance with the invention there is provided BCBO derivatives represented by the formulae (I) and (II): 
wherein R is xe2x80x94SH, xe2x80x94SO2Cl, xe2x80x94SO3H, N-substituted group or other electron-withdrawing or electron-donating groups; wherein X is a polyvalent organic bridging group.
In accordance with another aspect of the invention there is provided BCBO-containing copolymers represented by the formulae III-VI: 
wherein R is alkyl, aryl, haloalkyl, hydroxyalkyl, xe2x80x94CN, xe2x80x94CONH2, xe2x80x94Si(OCH3)3, 
hydroxyalkyl, cyanoalkyl, epoxyalkyl, alkoxy, ether, ester or acetate; and R1 is H or a methyl group.
In still another aspect of the invention there is provided BCBO-containing terpolymers represented by the formulae VII-X: 
wherein R is alkyl, aryl, haloalkyl, hydroxyalkyl, xe2x80x94CN, xe2x80x94CONH2, xe2x80x94Si(OCH3)3, 
hydroxyalkyl, cyanoalkyl, epoxyalkyl, alkoxy, ether, ester, acetate; R1 is H or methyl group; R3 is alkyl, aryl, or an ester moiety.
The alkyl radicals and moieties in the aforementioned definitions of R typically have 1 to 10, preferably 1 to 6, more preferably 1 to 4 carbon atoms.
The aryl radical in the aforementioned definitions of R typically has 6 to 10 carbon atoms, for example phenyl or naphthyl, and may be unsubstituted or substituted; typical substituents are one or more of alkyl, alkoxy and halogen, in which the alkyl and alkoxy have 1 to 10, preferably 1 to 6, more preferably 1 to 4 carbon atoms; and the halogen is fluorine, chlorine, bromine or iodine.
The values n and m in the above formulae identify the number of repeat units, each of n and m being in the range of 0.1 to 0.9. The designation  as in [] is convention for identifying a polymer backbone to which the pendant groups shown in the formulae are attached.
In general the copolymers and terpolymers of formula (III) to (X) have a weight average molecular weight of 3,000 to 100,000, preferably 3,000 to 60,000, more preferably 5,000 to 30,000.
In formula (III) n is less than 1; and in formula (VII) m/n and m+n should both be less than 1.
The present invention provides generally a new type of photo-crosslinker and photo-reactive polymers capable of undergoing grafting and crosslinking upon UV-Vis irradiation, without using any catalysts or photosensitizers. Specifically, the photochemistry based on the reaction of benzocyclobutenone with itself and alcohol is exploited, and the use of benzocyclobutenone as a latent reactive group in various polymers is developed therefrom.
Benzocyclobutenone is easily functionalized to give a series of benzocyclobutenone derivatives of formula (I)L 
wherein R is any electron-withdrawing and electron-donating group. Preferably, R is xe2x80x94SH, xe2x80x94SO2Cl, xe2x80x94SO3H, or N-substituted group, for example: 
The preparation of the functionalized BCBO derivatives (I-1 to I-6) uses 5-aminobenzocyclobutenone (U.S. Pat. No. 5,869,693). The functionalization is typically a reaction of 5-aminobenzocyclobutenone with 2-bromoethanol, acryloyl chloride, methacryloyl chloride, succinic anhydride or 3,5-dinitrobenzoyl chloride. Furthermore, direct chlorosulfonation of benzocyclobutenone affords 5-chlorosulfonyl benzocyclobutenone (I-7), which can be hydrolyzed to the corresponding sulfonic acid and reduced to the thiol. Compounds I-1 and I-2 undergo polymerization and crosslinking reaction in solution or in the solid state, as illustrated in Example 1, however useful polymerization and crosslinking reactions are not limited to those illustrated in Example 1, to yield polymeric resins upon photo irradiation at ambient temperatures. Compound I-3 is an acrylamide monomer, which is used to copolymerize with a variety of commercially available vinyl monomers to yield the corresponding photo-reactive vinyl polymers. Diamine I-6 is used to form a photo-reactive polyimide in the polycondensation of a diamine and a dianhydride. Both compounds I-4 and I-7 can be used to react with either the hydroxy or amino groups presented in a polymer, such as poly(4-hydroxy styrene), polyvinyl alcohol and polybutyral. Thus, the resulting grafted polymers contain a reactive BCBO group and can be photochemically crosslinked.
The BCBO derivatives can also be taken in the following formula II: 
wherein X is a polyvalent organic bridging group. Preferably, X is a divalent moiety selected from the group consisting of: 
The above bis-BCBO derivatives are generally prepared by reacting 5-aminobenzocyclobutenone with the corresponding diacid chloride, diisocyanate or dianhydride in solution, as described in U.S. Pat. No. 5,869,693. These bis-BCBO compounds are generally solids with high melting points and possess high thermal stability. Typically, the thermal ring-opening temperatures are over 200xc2x0 C. Thus, they can be used in melt mixing (e.g., melt extrusion or compression) with any hydroxy-containing polymers such as polyol, poly(vinyl alcohol) or polybutyral and presented in photocopy toner and as polymers for use as a pressure-sensitive adhesive that contains a small amount of the hydroxy functionality. These bis-BCBO compounds are typically photo crosslinkers for curing the above hydroxy-containing polymers, as illustrated in example 20.
Using BCBO-containing vinyl monomers such as I-3 or 5-aminobenzocyclobutenone, BCBO can thus be incorporated into a wide spectrum of vinyl polymers as a pendent group through copolymerization with vinyl monomers or through grafting onto anhydride-containing and carboxylic acid-containing polymers via imidization transformation and acid-amine coupling reactions. Thus, the BCBO-containing copolymers include those of the formulae III-VI above. For example, I-3 monomer can copolymerize with any commercial vinyl monomers such as styrene using AIBN (Azobisisobutyronitrile) as a radical initiator in solution. Furthermore, 5-aminobenzocyclobutenone may be grafted onto any maleic anhydride, or carboxyl acid containing commercial polymers. In addition, BCBO-containing polyimides or polyamides (or nylons) can also be prepared by the condensation of a diamine derived from BCBO such as I-6 with other dianhydrides and diacids or diacid chlorides, respectively. Preferably, the following BCBO-containing copolymers are prepared: 
The copolymers containing the BCBO moiety can be modified in bulk and at the surface through the photografting reaction of BCBO with small hydroxyl-containing molecules, oligomers and polymers. The BCBO photochemistry provides ways and means for tailoring and enhancing the bulk and/or the surface properties of existing polymers through grafting. The amount of the grafted component such as a polyol can be controlled by the BCBO content in the polymers. The BCBO content in the polymers is typically in the range of 0.5 mol % to 80 mol % and preferably about 5 mol %. Hydrophilic, hydrophobic, and other functional units with a hydroxyl end group can be linked onto the BCBO-modified commercial polymers simply upon UV irradiation.
The hydroxyl-containing functional groups, preferably, are hydrophilic poly(ethylene glycol) methyl ether (any molecular weight, more preferably, low molecular weight between 550-2000), hydrophobic Zonyl(copyright) fluoroalcohol, fluorescent pyrenebutanol, azo dye Disperse Red-1, electrochromic hydroxyl-end capped naphthalene imide, and conductive hydroxyl-end capped materials. By using functional hydroxyl-containing molecules, the bulk or surface of the polymers can be selectively modified to have desirable properties attributed to the functional hydroxyl groups that are grafted on. With respect to surface grafting, the UV curing can happen selectively on the selected area of the substrate, since the location of photo grafting can be chosen through a photomask or shadow mask, as illustrated in Example 13. The substrate can be any polymer films, either hydrophilic or hydrophobic.
Using BCBO derivatives such as I-3 and I-8, the terpolymers containing both BCBO and hydroxyl groups having the structural formulae VII-X are prepared. Preferably, BCBO and OH-containing terpolymers or precursors of such terpolymers include the following, having the repeat units n and m in a range of 0.1 to 0.9: 
For crosslinking in solution, the solvent used to dissolve the BCBO polymers can be any organic solvent except an alcohol and preferably is selected from tetrahydrofuiran, ethyl acetate, ether, dichloromethane, chloroform, 1,1,2,2-tetrachloroethane, N,N-dimethylformamide, hydrocarbons, toluene and xylenes. The polymer concentration can be very dilute (0.5%) to very concentrated (50%).
With respect to curing in the solid state, the size and thickness of polymer films can be made according to requirements, coating conditions, and UV source available. Preferably, the UV curing area can be between 0.1 mm2 to 15 cm2, the film thickness can be from 50 nanometers to 2 mm.
The UV curing temperature can be as low as 77xc2x0 K., and as high as 250xc2x0 C. (or just below the thermal ring opening temperature of the BCBO group in the said polymer). Preferably, the UV curing proceeds at ambient temperatures. The wavelength of the light source varies between 250 nm to 500 nm, depending on the chemical structure of BCBO derivatives. Preferably, a wavelength of 300 nm to 350 nm is selected. The UV curing time can be less than a second to a few hours, depending on the film thickness, light intensity and light dose.
As shown in Example 9, photo coupling reactions of benzocyclobutenone with a variety of alcohols gives the corresponding esters in almost 100% yield. Kinetic studies indicate that the photo reaction of BCBO follows the first order kinetic. Thus, the ring-opening of BCBO in the photo coupling reaction with alcohol is the rate-determining step. The same is true in the self dimerization or crosslinking reaction of BCBO in a polymer.
The self-curable polymers, which contain the BCBO group or contain both BCBO and hydroxyl groups, can form crosslinked structures upon UV irradiation in solution or in the solid state. The crosslinking degree is controlled by the BCBO content in the polymers. The BCBO content in the self-curable polymers is in the range of 0.5 mol % to 80 mol %, preferably, 5 mol %. The ratio of BCBO to hydroxyl moiety can be from 0.5:1 to 1:10. Preferably, the ratio is 1:1 to 1:3. The aforementioned UV curing conditions can also be applied here.
A polymer gel can be obtained from a solution of self-curable vinyl polymers, polyolefins, polyamides, and polyimides upon irradiation, indicating that these polymers can be used as photoresists to directly form patterns on the substrate. The substrate can be glass, silicon wafer, metal, mica, wood, ceramics or polymers. Preferably, the substrate is glass, polymer, mica or silicon wafer. The function of the self-curable BCBO polymers can be passive and active, preferably, conductive, electrochromic, photochromic, photovoltaic, non-linear optical or magnetic. Thus, the self-UV curable BCBO-containing polymers are very useful in microlithography, because of the fast and neat curing process, furthermore, there is no need for any photoinitiator, photo acid generator as currently used.
The self-UV curable BCBO-containing polymers can also form a new type of UV adhesive, as illustrated in Example 7, however formation of UV adhesives is not limited to Example 7. The BCBO content in the polymers is in the range of 0.5 mol % to 80 mol %, preferably, 1-5 mol %. The ratio of BCBO to hydroxyl moiety can be from 0.5:1 to 1:10. Preferably, the ratio is 1:1 to 1:3. The aforementioned UV curing conditions can also be applied here. The shear modulus of the polymer containing only 5 mol % of BCBO was determined and significantly enhanced after UV curing.
The self-curable BCBO-containing polymers and oligomers, as described and demonstrated in Examples 1, 6, 7, 10, 12 and 16, can be potentially used as adhesives and sealants for microelectronics packaging and interconnection of optical fibers.